NOx emissions from vehicles with internal combustion engines are an environmental problem recognized worldwide. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from vehicles. Manufacturers and researchers have put considerable effort toward meeting that regulation. NOx emissions can be controlled in conventional gasoline powered vehicles, which use stoichiometric fuel-air mixtures, by three-way catalysts. In the absence of oxygen, three-way catalysts reduce NOx by reaction with CO and unburned hydrocarbons. In diesel powered vehicles and lean-burn gasoline engines, however, the exhaust is too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions in diesel-powered vehicles. One set of approaches focuses on the engine. NOx is generated primarily at high temperatures. By limiting the adiabatic flame temperature, through exhaust gas recirculation (EGR) for example, NOx production can be reduced. Lowering the adiabatic flame temperature to eliminate NOx production, however, causes engine efficiency to decrease and smoke to appear in the exhaust. It is commonly believed that there is a trade-off between NOx production and particulate matter production in diesel engines. It is less well known that if the adiabatic flame temperature is dropped sufficiently, particulate matter production will also decrease. In any event, clean combustion cannot be achieved solely by varying the adiabatic flame temperature at which a diesel engine operates.
One way to reduce total combustion byproducts is to homogenize fuel air mixtures in diesel engines. This can be accomplished by mixing fuel with air prior to injection or injecting all or part of the fuel into an engine cylinder before or early in a compression stroke. While studies show a reduction in emissions, this approach has not been proven commercially and does not eliminate diesel combustion byproducts altogether.
Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOX catalysts, NOX adsorber-catalysts, and selective catalytic reduction (SCR). Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiency with lean-burn catalysts is unacceptably low.
NOx adsorber-catalysts alternately adsorb NOx and catalytically reduce it. The adsorber can be taken offline during regeneration and a reducing atmosphere provided. The adsorbant is generally an alkaline earth oxide adsorbant, such as BaCO3 and the catalyst can be a precious metal, such as Ru.
SCR involves using ammonia as the reductant. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuously into the exhaust. SCR can achieve NOx reductions in excess of 90%, however, there is concern over the lack of infrastructure for distributing ammonia or a suitable precursor. SCR also raises concerns relating to the possible release of ammonia into the environment.
An alternative approach to reducing emissions is to convert the chemical energy of the fuel into electrical energy using a fuel cell. Fuel cells are not very effective at extracting power from long chain hydrocarbons, but fuel reformers can be used to break long chain hydrocarbons into smaller more reactive molecules such as short chain hydrocarbons, oxygenated hydrocarbons, hydrogen, and carbon monoxide, which are suitable fuels for a fuel cell. For example, U.S. Pat. No. 5,678,647 suggests powering a fuel cell for a vehicle drive system using a conventional fuel processed through a reformer. The reformer and the fuel cell must be heated before they are operative to produce useful power.
U.S. Pat. No. 6,276,473 describes a hybrid power generation system comprising an engine, a fuel reformer, and a fuel cell. The engine is used to provide cold start-power and the engine's exhaust is used to heat the fuel reformer and the fuel cell. When the reformer and fuel cell reach their operating temperatures, the reformer/fuel cell system begins to produce power. The engine can continue to operate after warm-up or be turned off.
U.S. Pat. No. 6,655,325 describes a power generation system comprising an internal combustion engine and a fuel cell. The engine can operate as a reformer and provides fuel for the fuel cell. The engine can also provide shaft power, or alternatively all the shaft power can be derived from the fuel cell. Particulate matter in the engine exhaust is said to be removed by the fuel cell and a catalytic converter. It is also suggested that by treating the fuel cell exhaust with a catalytic converter, near zero emissions of hydrocarbons and nitric oxide can be achieved.
In spite of progress, there remains a long felt need for environmentally friendly, efficient, and reliable power generation systems for vehicles.